1. Field of the Invention
This invention relates to a valve timing control system for an internal combustion engine, which controls timing for closing a valve opened by a cam provided on a camshaft of the engine by temporarily holding the valve by an actuator.
2. Description of the Prior Art
Conventionally, a valve timing control system of this kind has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 63-289208. This valve timing control system opens and closes engine valves by cams provided on a camshaft via rocker arms, and includes holding mechanisms for holding the engine valves in respective open positions. The holding mechanisms are each implemented by a solenoid actuator comprised of a solenoid fixed to the cylinder head and an armature fixed to a valve stem of an engine valve. The energization of the coil of the solenoid is controlled by a control unit. The armature is arranged in a manner opposed to the solenoid such that when the engine valve is actuated to the open position by the cam, there is a slight spacing between the armature and the solenoid. When the engine valve reaches the open position, the solenoid is energized in a manner dependent on an operating condition of the engine, whereby an attractive force of the solenoid is exerted on the armature to hold the engine valve in the open position over a predetermined time period corresponding to duration of the energization. Thus, the timing for closing the engine valve is delayed, i.e. the valve-closing timing is controlled.
In the conventional valve timing control system, however, there occurs a response delay between a time an instruction is delivered for holding the engine valve and a time a holding operation is actually carried out on the engine valve. The response delay makes it difficult to hold the engine valve in desired timing. Particularly, in this valve timing control system, the solenoid actuator is driven when the engine valve reaches the open position by the operation of the cam, and therefore, when the operating condition changes, there is a fear that the engine valve cannot be held in desired timing due to the delayed response of the solenoid actuator, making it impossible to achieve a desired valve lift curve or even hold the engine valve. In such a case, the combustion state is degraded to adversely affect exhaust emissions. Particularly, the response of the solenoid actuator is delayed by a time period the magnetic flux takes to rise. Further, the rise of the magnetic flux becomes slower as the power supply voltage is lower, and becomes relatively slower with respect to the operating speed of the engine valve as the engine rotational speed is higher. This increases the possibility of failure in holding the engine valve. Further, if a hydraulic actuator is employed for the mechanism for holding the engine valve, instead of the solenoid actuator, the rise in the hydraulic pressure becomes slower as the oil temperature is lower. Further, as the engine rotational speed is higher, the response of the holding mechanism becomes slower, which can also increase the possibility of failure in holding the engine valve.
It is an object of the invention to provide a valve timing control system for an internal combustion engine, which is capable of properly holding a valve in predetermined holding timing by an actuator. It is a further object of the invention to provide a valve timing control system for an internal combustion engine, which can attain energy saving by efficient operation of the actuator, when the actuator is formed by a solenoid actuator.
To attain the above object, the invention provides a valve timing control system for an internal combustion engine, for controlling valve-closing timing of a valve opened by a cam provided on a camshaft, by temporarily holding the valve,
the valve timing control system comprising:
an actuator for holding the valve;
response delay-predicting means for predicting a response delay of the actuator by a response delay prediction value;
output timing-setting means for setting output timing in which a drive signal for driving the actuator is output, according to the predicted response delay prediction value; and
holding timing control means for controlling holding timing in which the valve is held by the actuator, by outputting the drive signal to the actuator, based on the set output timing.
According to this valve timing control system, the response delay of the actuator is predicted by a response delay prediction value, and output timing in which the drive signal for driving the actuator is output is set according to the predicted response delay prediction value. Further, holding timing in which the valve is held by the actuator is controlled by outputting the drive signal to the actuator, based on the set output timing. Therefore, the operation of the actuator can be started in proper timing dependent on the predicted response delay of the actuator, which makes it possible to properly hold the valve in predetermined appropriate holding timing while compensating for the response delay of the actuator and enabling efficient operation of the actuator.
Preferably, the valve timing control system further comprises operating condition-detecting means for detecting an operating condition of the engine, and the response delay-predicting means predicts the response delay of the actuator according to the detected operating condition of the engine.
According to this preferred embodiment, it is possible to predict the response delay of the actuator according to the detected operating condition of the engine. Therefore, the operation of the actuator can be started in appropriate timing dependent on actual operating conditions of the engine, which makes it possible to properly hold the valve in predetermined holding timing while causing the actuator to efficiently operate without delay in operation.
More preferably, the operating condition-detecting means includes rotational speed-detecting means for detecting a rotational speed of the engine as the operating condition of the engine, and the response delay-predicting means sets the response delay prediction value to a larger value as the detected rotational speed of the engine is higher.
According to this preferred embodiment, as the rotational speed of the engine is higher, the operation of the actuator is started earlier, and hence even when the engine is in a high rotational speed condition, the valve can be more properly held without causing relative delay in operation of the actuator in spite of a high operating speed of the valve.
Preferably, the valve timing control system further comprises drive source condition-detecting means for detecting a condition of a drive source of the actuator, and the response delay-predicting means predicts the response delay of the actuator, according to the detected condition of the drive source.
As described hereinbefore, when the actuator is a solenoid actuator, the rise of the magnetic flux of the electromagnet of the actuator varies depending on the voltage of the power supply, while when the actuator is a hydraulic actuator, the rise of the oil pressure varies depending on the oil temperature of an oil pressure source. Thus, the rise time or start of the actuator varies depending on the condition of the drive source. According to this preferred embodiment, it is possible to predict the response delay of the actuator according to the detected condition of the drive source, thereby starting the operation of the actuator in appropriate timing dependent on the actual condition of the drive source.
More preferably, the actuator is formed by a solenoid actuator, and the drive source condition-detecting means includes power supply voltage-detecting means for detecting a voltage of a power source of the solenoid actuator, as the condition of the drive source, the response delay-predicting means setting the response delay prediction value to a larger value as the detected voltage of the power source is lower.
According to this preferred embodiment, when the actuator is a solenoid actuator, the operation of the solenoid actuator is started earlier as the voltage of the power source is lower. This makes it possible to hold the valve in predetermined appropriate holding timing without delay in operation of the solenoid actuator, even when the voltage of the power source is low.
Further preferably, the solenoid actuator includes an armature that is moved to follow motion of the valve when the valve is lifted by the cam in a valve-opening direction, and an electromagnet that is energized when the armature is close thereto, by electric power supplied as the drive signal from the power source, to thereby attract the armature thereto to hold the valve, and the holding timing control means controls the electric power supplied to the electromagnet by constant voltage before the valve is held, and by constant current after the valve is held.
When a valve is actuated by a cam in the valve-opening direction, the valve displacement speed can be made slower by a disturbance, such as frictional resistance and biting of wear particles, causing a decrease in the lift of the valve, which makes it impossible to obtain predetermined lifting timing. On the other hand, this preferred embodiment of the invention is configured such that when the valve is opened, the valve is held by causing the armature following the motion of the valve to be attracted to the electromagnet, and hence, it is necessary for the armature to be close to the electromagnet when the holding of the valve is executed. Therefore, in case a decrease in the valve lift occurs owing to such a disturbance described above, the armature can be positioned too far from the electromagnet when the valve is to be held, which makes it impossible for the electromagnet, which is energized at this time, to attract the armature thereto, resulting in an error in holding of the valve (loss of synchronization).
On the other hand, the inductance L of the coil of the electromagnet is expressed by the equation: L=Nxc2x7xcex94xc3x8/xcex94i (N: the number of windings of the coil; xc3x8: magnetic flux; i: electric current). Therefore, as the distance between the armature and the electromagnet is smaller, the inductance L is larger. Further, the electric current i is expressed by the equation: i=E/R(1xe2x88x92exp(xe2x88x92R/Lxc2x7t)) (E: power supply voltage; R: resistance of the coil), and finally converges to a value of E/R. A converging time period over which the electric current converges to the value of E/R is larger as the inductance L is larger.
From the relationship described above, when a decrease in the valve lift occurs due to the disturbance, the distance between the armature and the electromagnet becomes larger than usual, resulting in a decreased value of the inductance L. Accordingly, the converging time period over which the electric current i converges is shortened to cause the current to flow more easily to increase the current i flowing through the coil of the electromagnet. As a result, a larger attractive force than usual acts on the armature, so that even if the armature is far from the electromagnet to some extent, it can be properly attracted to the electromagnet.
Therefore, as in the case of this preferred embodiment, if the energization of the electromagnet is controlled by constant voltage before the valve is held, it is possible to allow an increase in the current i which becomes easier to flow. As a result, the attractive force of the electromagnet is increased, so that the armature can be attracted to the electromagnet even if the armature is far from the electromagnet to some extent, whereby the valve can be positively held. Thus, by supplying over excitation current to the electromagnet by constant-voltage control before the valve is held, the valve timing control system can be made tough against the disturbance, whereby the valve can be held in a further appropriate manner. In contrast, if constant-current control is carried out before the valve is held, the current is limited so as to allow only a predetermined or lower amount of current to flow, so that if the armature is not within a predetermined distance of the electromagnet due to a decreased valve lift caused by the disturbance, there is a fear that the failure of holding of the valve can occur.
Further, once the valve is held, the armature is attracted at the electromagnet so that the distance between the two becomes constant. Therefore, in this state, by controlling the energization by constant current (holding current), it is possible to continue positive holding of the valve and at the same time reduce the power consumption.
Preferably, the response delay-predicting means calculates an output start offset time period by which a start of output of the drive signal to the actuator is shifted, as the response delay prediction value, and the output timing-setting means includes an output start timer that counts up to a time going back from a reference time corresponding to a predetermined reference crank angle position by the output start offset time period, thereby causing the drive signal to start to be output to the actuator at the time.
According to this preferred embodiment, the output start offset time period is calculated as the response delay prediction value, and the output start timer counts up to a time going back from a reference time corresponding to a predetermined reference crank angle position by the output start offset time period, thereby causing the drive signal to start to be output to the actuator at the time. This makes it possible to cause the drive signal to start to be delivered in appropriate timing with accuracy, in synchronism with the rotation of the cam, and cause the operation for holding the valve to be properly completed by the time the reference crank angle position is reached.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.